1. Field of the Invention
The present invention relates to a novel nickel powder suitable for a thick film conductor paste and a process for preparing the same, and also to a conductor paste using the nickel powder and a multilayer electronic component or a multilayer substrate comprising a conductor formed using the paste.
2. Description of the Prior Art
In the field of electronics, thick film pastes, such as conductor pastes and resistor pastes, have been used for the production of electronic circuits and components, such as resistors, capacitors, and IC packages. The thick film paste is prepared by homogeneously mixing and dispersing a conductive powder, such as a metal, an alloy, or metal oxide, optionally together with a vitreous binder or other additive(s), in an organic vehicle to prepare a paste. The paste, after application onto a substrate, is fired at a high temperature to form a conductor film or a resistor film.
Multilayer ceramic electronic components, such as multilayer capacitors and multilayer inductors, and multilayer ceramic substrates are generally prepared by alternately laminating an unfired ceramic green sheet, made of, for example, a dielectric material or a magnetic material, and an internal conductor paste layer to form a plurality of layers and co-firing the plurality of layers. Noble metals, such as palladium, silver-palladium, and platinum, have hitherto been mainly used as the internal conductor. In recent years, however, base metals, such as nickel, have attracted attention due to a demand for saving resources and the solution of problems, such as delamination, cracking and other unfavorable phenomena attributable to oxidation expansion during firing of palladium or silver-palladium.
In these multilayer components and multilayer substrates, there is a tendency that the number of layers constituting the multilayer structure is increased. For example, a multilayer capacitor having a multilayer structure of several hundred layers has been prepared. This has led to a demand for a reduction in thickness of each ceramic layer and a further reduction in thickness of each internal conductor layer. For example, when the thickness of the ceramic layer is about 3 .mu.m, an internal conductor thickness of not more than 1 .mu.m, preferably about 0.5 .mu.m, is required because when the thickness is larger than the above value, the center portion of the laminate is thick,leading to the creation of structural defects and deteriorated reliability.
The conventional conductor paste using a nickel powder, however, has a problem that excessive sintering in the step of firing results in the formation of an internal conductor of a discontinuous film, leading to an increased resistance value and breaking and, in addition, aggregation of the nickel powder results in an increased thickness of the conductor, limiting the reduction of the thickness of the film. Specifically, the nickel powder, particularly when fired in an inert atmosphere or a reducing atmosphere for preventing oxidization of the nickel powder, initiates sintering in an early stage, that is, initiates sintering and shrinking at a low temperature of 400.degree. C. or below, even in the case of a single crystal powder having a relatively low activity. On the other hand, the temperature at which the ceramic layer initiates sintering is generally much above this temperature, for example, about 1,200.degree. C. in the case of barium titanate. Therefore, the nickel and the ceramic layer, even when co-fired, do not shrink simultaneously, and the nickel film is pulled toward the face. Therefore, it is considered that small voids created in the nickel film in the course of sintering at a relatively low temperature are likely to become large with the progress of the sintering in a high temperature region and that the aggregation of the nickel powder causes the film to be grown in the thicknesswise direction.
For this reason, what is needed for achieving the reduction in the thickness of the internal conductor layer of nickel is considered to further reduce the size of the nickel powder and to improve the dispersibility of the nickel powder to minimize the creation of voids during sintering and, in addition, to allow the sintering-shrinking behavior of the nickel layer to coincide with that of the ceramic layer.
Further, the uncoincidental sintering-shrinking behavior between the conductor layer and the ceramic layer, particularly when the film thickness is large, is causative of the creation of structural defects such as delamination and cracking, resulting in a lowering in yield and reliability.
An increase in the resistance value and creation of structural defects attributable to similar excessive sintering raise a problem also in the co-firing of an external conductor and the above layers.
Various studies have hitherto been made with a view of retarding the sintering of the conductors until the temperature reaches the sintering initiation temperature of the ceramic layer.
Examples of the proposed methods include the addition of metal oxide particles, such as particles of titanium oxide, zirconium oxide, barium oxide, or iron oxide or a precursor compound capable of producing the above oxide during firing to the conductor paste, or coating of the metal powder with the above metal oxide particles or precursor thereof. The above oxide is an insulator, and the addition thereof in a large amount results in a lowering in conductivity.
Further, the addition of a large amount of a ceramic powder having the same composition as that used in the ceramic layer can apparently delay the initiation of the shrinking of the conductor film to about 800.degree. C. In this case, however, since sintering of the metal powder per se is not inhibited, sintering at a high temperature of about 1,300.degree. C. results in loss of continuity and conductivity of the conductor film.